Cyclic process for producing an alkali solution of cellulose carbamate and precipitating the carbamate

Abstract

The present invention concerns a cyclic process for producing an alkali aqueous solution of cellulose carbamate, for precipitating the cellulose carbamate from the alkali solution and for recovering and recycling the remaining chemicals. The process comprises the following steps: (a) an alkali solution of cellulose carbamate is prepared by dissolving cellulose carbamate in an aqueous solution of sodium hydroxide, (b) the solution obtained in step (a) is contacted with a precipitation aqueous solution containing sodium carbonate, whereby the cellulose carbamate is precipitated and can be removed, (c) from the solution in step (b) sodium carbonate is crystallized and is returned to the precipitant solution in step (b), and (d) mother liquor containing sodium hydroxide, obtained, i.e. remaining from, step (c), is returned to step (a) for dissolving cellulose carbamate. The crystallizing in step (c) is accomplished in two or several steps by cooling.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims. 
     
       1. A cyclic method of producing an alkali solution of cellulose carbamate, precipitating the cellulose carbamate in the form of fiber or film from the alkali solution, and recovering and converting the chemicals remaining after precipitation of the cellulose carbonate for re-use in the method, comprising the steps of: (a) dissolving cellulose carbamate in an aqueous solution of sodium hydroxide, thereby producing an alkali solution of cellulose carbamate;   (b) extruding said alkali solution into contact with an aqueous precipitating solution containing sodium carbonate, whereby the cellulose carbamate precipitates and can be separated from the resultant solution;   (c) withdrawing the resultant solution of step (b) and separating said solution into aqueous sodium hydroxide and sodium carbonate components;   (d) recycling the aqueous sodium hydroxide obtained in step (c) to dissolve cellulose carbamate according to step (a);   (e) recycling the sodium carbonate obtained in step (c) to regenerate the precipitating solution utilized in step (b).   
     
     
       2. A method according to claim 1, wherein said precipitating solution is substantially free of mineral acid. 
     
     
       3. A method according to claim 1, wherein said precipitating solution contains from about 10 to about 30% by weight of sodium carbonate. 
     
     
       4. A method according to claim 3, wherein said precipitating solution contains from about 20 to about 25% by weight of sodium carbonate. 
     
     
       5. A method according to claim 1, wherein said precipitating solution contains a maximum of about 6% by weight of sodium hydroxide. 
     
     
       6. A method according to claim 5, wherein said precipitating solution contains a maximum of about 3% by weight of sodium hydroxide. 
     
     
       7. A method according to claim 1, wherein said alkali solution contains from about 4 to about 12% by weight of cellulose carbamate. 
     
     
       8. A method according to claim 7, wherein said alkali solution contains from about 5 to about 12% by weight of sodium hydroxide. 
     
     
       9. A method according to claim 8 wherein said alkali solution contains about 10% by weight of sodium hydroxide. 
     
     
       10. A method according to claim 1, wherein said alkali solution is extruded through spinnerettes into the precipitating solution, thereby causing the precipitation of cellulose carbamate in fiber form. 
     
     
       11. A method according to claim 1, wherein said alkali solution is extruded through slit means into the precipitating solution, thereby causing the precipitation of cellulose carbamate in film form. 
     
     
       12. A method according to claim 1, wherein the separation of sodium carbonate from the aqueous sodium hydroxide present in the solution obtained in step (b) is accomplished by crystallization of the sodium carbonate out of the solution. 
     
     
       13. A method according to claim 12 wherein said crystallization is accomplished by cooling the solution with the aid of cooling means. 
     
     
       14. A method according to claim 13 wherein said crystallization is accomplished by cooling the solution in at least two stages, during the first of which the solution is cooled to a selected temperature and crystallization is permitted to occur, and during each succeeding stage the solution is cooled to temperatures successively lower than said selected temperature, with additional crystallization occurring at each stage. 
     
     
       15. A method according to claim 13 wherein said solution is cooled to a final crystallization temperature of -5° C. 
     
     
       16. A method according to claim 12 wherein the sodium carbonate is separated after crystallization from the aqueous sodium hydroxide by vacuum filtration.